[cgal] Eigen3 feature support

[nyoma-diamond]

Is your feature request related to a problem? Please describe.

A very large portion of functionality available via CGAL is dependent on Eigen3. For reference, see https://doc.cgal.org/latest/Manual/thirdparty.html#title8 and https://doc.cgal.org/latest/Manual/packages.html.

Currently, there is no feature of the port to properly Eigen3 support.
There is a Qt feature which requests Eigen3, but this is excessive for uses that don't require Qt (Also, it's unclear why the Qt feature even requests Eigen3 at all, as CGAL::Qt doesn't depend on Eigen3 based on https://doc.cgal.org/latest/GraphicsView/ and https://doc.cgal.org/latest/Manual/packages.html. The feature seems excessively broad / overspecified).

Proposed solution

At the bare minimum, an optional eigen feature should be added to the CGAL port. Currently the consumer is required to manually request Eigen3 separately which isn't entirely clear.

Describe alternatives you've considered

As of right now, it's possible to work around this by manually requesting Eigen3 and including it in parallel with CGAL. It's a straightforward solution, but unusual to need to perform manually if Eigen is just a transitive dependency of CGAL and isn't used by the consuming project.

Additional context

No response

[nyoma-diamond]

As an additional side note regarding the overspecified qt optional feature: Upon investigation of CGAL's GraphicsView module and basic Qt viewer, the qt feature can likely be better specified as requiring qtbase with the optional features widgets, gui, and opengl (note that qtsvg is optional. CGAL only uses it optionaly in a few locations and has switches via #if QT_SVG_LIB to compile without qtsvg if not available). I'm not sure whether qtdeclarative is required, but I don't believe it is since neither QML nor QtQuick are used outside of demos to my knowledge.

Note that according to CGAL's documentation:

The exhaustive list of Qt6 components used in demos is: Core, Gui, Help, OpenGL, OpenGLWidgets, Qml, Svg, Widgets, WebSockets, Network, and qcollectiongenerator (with sqlite driver plugin).

However many of these are explicitly for demos, and not actually dependencies of CGAL algorithms themselves.